The disclosure relates to novel methods for determining polytetrafluoroethylene (PTFE) or other filler or particulate concentration in dispersions, such as dispersions used in the manufacture of electrophotographic devices. More in particular, the disclosure relates to novel methods of determining the PTFE or other filler or particulate concentration of charge transport layer (CTL) and anti-curl back coating (ACBC) layer dispersions using centrifugation and drying methods.
In the art of electrophotography, an electrophotographic device comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the surface of the photoconductive insulating layer. The device is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles, for example from a developer composition, on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper.
Electrophotographic imaging members are usually multilayered photoreceptors. U.S. Pat. No. 5,725,983, incorporated herein by reference in its entirety, describes an electrophotographic imaging member including a supporting substrate having an electrically conductive layer, a hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, a ground strip layer and an optional overcoating layer, at least one of the charge transport layer, ground strip layer and the overcoating layer comprising a blend of inorganic and organic particles homogeneously distributed in a film forming matrix, the inorganic particles and organic particles having a particle diameter less than about 4.5 micrometers. These electrophotographic imaging members may have a flexible belt form or rigid drum configuration. For most multilayered flexible photoreceptor belts, an anti-curl layer is usually employed on the back side of the substrate support, opposite to the side carrying the electrically active layers, to achieve the desired photoreceptor flatness.
Examples of photosensitive members having at least a separate charge generation layer (CGL) and charge transport layer are disclosed in U.S. Pat. Nos. 4,265,990, 4,233,384, 4,306,008, 4,299,897 and 4,439,507. The disclosures of these patents are incorporated herein in their entirety. The charge generating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer. This photogenerating layer includes, for example, inorganic photoconductive particles or organic photoconductive particles dispersed in a film forming polymeric binder.
The charge transport layer generally includes at least one charge transport material. Any suitable charge transport molecule known in the art may be used, and the charge transport molecules may either be dispersed in the polymer binder or incorporated into the chain of the polymer. Preferably, the charge transport material comprises an aromatic amine compound. More preferably, the charge transport layer comprises an arylamine small molecule dissolved or molecularly dispersed in the binder. Typical aromatic amine compounds include triphenyl amines, bis and poly triarylamines, bis arylamine ethers, bis alkyl-arylamines and the like. U.S. Pat. No. 6,337,166, incorporated herein by reference in its entirety, describes suitable binders including polycarbonates, polyesters, including polyethylene terephthalate, polyurethanes, polystyrenes, polybutadienes, polysulfones, polyarylethers, polyarylsulfones, polyethersulfones, polycarbonates, polyethylenes, polypropylenes, polymethylpentenes, polyphenylene sulfides, polyvinyl acetates, polyvinylbutyrals, polysiloxanes, polyacrylates, polyvinyl acetals, polyamides, polyimides, amino resins, phenylene oxide resins, terephthalic acid resins, phenoxy resins, epoxy resins, phenolic resins, polystyrene and acrylonitrile copolymers, polyvinylchlorides, polyvinyl alcohols, poly-N-vinylpyrrolidinone)s, vinylchloride and vinyl acetate copolymers, acrylate copolymers, alkyd resins, cellulosic film formers, poly(amideimide), styrene-butadiene copolymers, vinylidenechloride-vinylchloride copolymers, vinylacetate-vinylidenechloride copolymers, styrene-alkyd resins, polyvinylcarbazoles, and the like. These polymers may be block, random or alternating copolymers. Additional additives, such as antioxidants or leveling agents, may be included in the charge transport layer material as needed or desired. The solvent system comprises of at least tetrahydrofuran (THF). Other or alternative solvents may also be present, if desired.
Conventional charge transport layers without additives suffer from a fast, nearly catastrophic wear rate of 8 to 10 microns or more per 100 kilocycles when the photoreceptor is charged using an AC bias charging roll (BCR). The use of AC bias charging rolls to charge a photoreceptor surface is conventional in the art for forming images in low speed, for example up to 40 ppm, imaging devices (e.g., copiers and printers). However, the corona generated from the AC current, applied to the BCR, decomposes on the top photoreceptor layer. The decomposed material can be easily removed by a cleaning blade. Such a repeated process during the printing cycle wears out the photoreceptor top layer very quickly.
Wear rate is a significant property in that it limits the life of the photoreceptor, and photoreceptor replacement in electrostatographic devices such as copiers and printers is very expensive. It is thus very significant to limit wear of the photoreceptor so as to achieve a long life photoreceptor, particularly with respect to small diameter organic photoreceptor drums typically used in low speed copiers and printers that are charged with an AC BCR. In such small diameter drums, 100 kilocycles translates into as few as 10,000 prints. CTL wear results in a considerable reduction in device sensitivity, which is a major problem in office copiers and printers that typically do not employ exposure control. In addition, the rapid wear of the top photoreceptor layer requires better cleaning of the debris from the photoreceptor surface in order to maintain good toner transfer and good copy quality.
U.S. Pat. No. 5,096,795, incorporated herein by reference in its entirety, describes an electrophotographic imaging member comprising a charge transport layer comprised of a thermoplastic film forming binder, aromatic amine charge transport molecules and a homogeneous dispersion of at least one of organic and inorganic particles having a particle diameter less than about 4.5 micrometers, the particles comprising a material selected from the group consisting of microcrystalline silica, ground glass, synthetic glass spheres, diamond, corundum, topaz, polytetrafluoroethylene, and waxy polyethylene, wherein said particles do not decrease the optical transmittancy or photoelectric functioning of the layer. The particles provide coefficient of surface contact friction reduction, increased wear resistance, and durability against tensile cracking without adversely affecting the optical and electrical properties of the imaging member.
Thus, it has been broadly known to attempt to utilize small particles such as polytetrafluoroethylene (PTFE) in outer layers of a photoreceptor in an effort to increase the cleanability and durability of the outer photoreceptor layers. PTFE particles may be incorporated into the dispersion along with a surfactant. Any commercially available PTFE particle may be employed, including, for example, MP1100 and MP1500 from Dupont Chemical and L2 and L4, Luboron from Daikin Industry Ltd., Japan. The size of the PTFE particles are preferably less than 2 micron diameter, most preferably less than 0.3 micron. Preferably, the surfactant is a fluorine-containing polymeric surfactant. Most preferably, the fluorine-containing polymeric surfactant is a fluorine graft copolymer, for example GF-300 available from Daikin Industries. These types of fluorine-containing polymeric surfactants are described in, for example, U.S. Pat. No. 5,637,142, incorporated herein by reference in its entirety.
Filler or particulate materials, such as PTFE, are also known to be used in other layers of a photoreceptor, including in an anti-curl backing layer and/or in a ground strip layer. For example, U.S. Pat. No. 6,303,254 describes an electrostatographic imaging member including: a flexible supporting substrate; an imaging layer having an optional adjacent ground strip layer coated on one side of the substrate; and an anti-curl backing layer coated on the other side of the substrate which layer is comprised of a film forming polymer binder, an optional adhesion promoting polymer, and a dispersion of polytetrafluoroethylene particles which dispersion has particles with a narrow diameter particle size distribution of from about 0.19 micrometer to about 0.21 micrometer, and an average diameter particle size of about 0.20 micrometer. The optional ground strip layer can include the same dispersion of polytetrafluoroethylene particles as the anti-curl backing layer.
Particles such as polytetrafluoroethylene tend to aggregate and/or slowly settle over time in a CTL coating dispersion as a result of the inherent instabilities of current formulations and the inadequacy of the surfactants to completely stabilize the system. Thus, it is necessary to frequently stir the dispersion in order to avoid settling of the PTFE particles. Moreover, manufacturing processes involve the transport of the CTL through piping and filters that can unpredictably change the concentrations of PTFE.
Because reproducible thorough dispersion of PTFE can often prove difficult to obtain or maintain, it is important to accurately measure PTFE concentration regardless of particle size or uniformity of dispersion. In this regard, various methods to measure PTFE concentration exist. These methods include Differential Scanning Calorimetry (DSC) and light scattering.
The DSC method provides quantitative measurements of instantaneous heat capacities. By correlating heat capacity to weight of a standard sample, a relative concentration measurement can be obtained for PTFE. The shortcomings of this method are that DSC is sensitive to crystallinity and particle size. Theoretically, crystallinity can be accounted for only by specifically measuring each lot of PTFE for calibration. Moreover, DSC readings can also be affected by particle sizes and size distribution, thereby skewing heat capacity, and hence concentration, values. The magnitudes of these sources of error are unknown. This method continues to be a relatively expensive and time-consuming test to perform.
U.S. Pat. No. 6,326,111, incorporated herein by reference in its entirety, describe light scattering. In the light scattering method a small amount of the dispersion is added into a solvent mixture in a cell used for light scattering measurement. The solvent mixture has the same composition as the one used for dispersion. The solution is then mixed and sonicated to let the dispersion uniformly mix into the solvents. The cell is then put into the light scattering instrument for measurement. This measurement is strongly affected by particle sizes and particle size distributions. Even a few large particles in the testing material, such as from undermilling, aggregation or impurities, will dominate the scattering light signal. In principle, light scattering can be used to determine the PTFE concentration only if the unknown dispersion and all standard dispersions for the calibration curve are monodispersed with similar particle sizes and shapes, if the particle concentration is the only difference between the known dispersion and all standard dispersions for calibration (such as viscosity, temperature, refractive index) and if the concentrations of all dispersions are kept reasonably low to avoid multi-scattering.
These methods do not capture the complexity of PTFE-containing dispersions, which can have different distributions of particle size and can be non-uniform. In this regard, the utilization of the aforementioned methods involves the assumption that all of the solutions are uniform dispersions of single size particles.
This measurement problem becomes apparent in analyzing well-dispersed PTFE systems, with calibration to a poorly dispersed PTFE system. The usage of an original calibration curve developed with a series of dispersions of different particle size/distribution, coupled with a well-dispersed system of smaller particle size can cause DSC and light scattering techniques to register a much lower PTFE concentration than actuality.
One way to reduce the error is to generate unique calibration curves for each batch of dispersion to help account for PTFE lot and milling variations. However, this is impractical in practice. What is still desired, then, is a reliable method for measuring PTFE concentration regardless of uniformity of dispersion or particle size.